Liquid nitriding process using urea



United States Patent 3,303,063 LIQUID NITRIDIN G PROCESS USING UREA Robert J. Pietryka, Warren, Mich., assignor to General The present invention relates generally to a process for case hardening ferrous alloy articles to improve the fatigue life and wear resistance of the articles. More particularly, the present invention relates to a low cost liquid nitriding process wherein urea is used in a fused salt bath to provide the source of nascent nitrogen in the bath.

It is the principal object of the present invention to provide a liquid nitriding process to improve the wear resistance and fatigue life of ferrous alloy articles which is sub stantially less expensive to employ than conventional liquid nitriding processes.

It is another object of the present invention to provide a unique and relatively inexpensive method of making a liquid nitriding salt bath composition for case hardening ferrous alloy articles.

These and other objects are accomplished in accordance with the present invention by adding urea to salt mixtures containing alkali and/or alkaline earth metal cyanide and/or carbonate salts which will react upon heating with the thermal decomposition products of urea to form cyanate compounds, heating the urea and the salt mixture to a suitable temperature to provide a fused salt nitriding bath containing these cyanate compounds which function as the nitriding agent in the bath, immersing the ferrous alloy article in the fused salt bath for a sufficient time and at a suitable temperature to form a nitride case on the article, and subsequently removing the article from the bath and cooling it, thereby providing a wear resistant, compressively stressed nitride case on the article.

Liquid nitriding of a ferrous alloy article to improve its fatigue life and wear resistance has conventionally been carried out by using a fused salt bath consisting of a mixture of alkali metal cyanates with other salts, such as alkali metal cyanides, chlorides and carbonates, immersing the article in the bath for a sufficient time and at a suitable temperature to provide a nitride case thereon and subsequently removing the article from the bath and cooling it. As exemplified by United States Patent No. 2,875,095 Miiller, conventional liquid nitriding salt baths have heretofore employed relatively large amounts of 3,303,063 Patented Feb. 7, 1967 7 ice of the bath compositions. Moreover, in many instances, the resulting case hardening effect which may be imparted to a ferrous alloy article by the subject liquid nitriding process provides a greater improvement in wear resistance and fatigue life on a wide variety of ferrous alloy articles, such as a mild steel gear or steel automobile crankshaft, than is normally obtained using conventional liquid nitrid ing and other gaseous nitriding processes using ammonia.

Other objects, features and advantages of the present invention will be apparent from the following detailed description of certain embodiments and specific examples thereof.

the comparatively expensive cyanate salts, which serve as the active of nascent nitrogen or nitriding agent in the baths. However, the use of these cyanate salts contributes substantially to the overall cost of conventional liquid nitriding processes which has precluded the use of liquid nitriding in many applications.

Since urea, cyanide salts and carbonate salts are substantially less expensive than the cyanate salts previously employed in making up conventional liquid nitriding baths, the overall cost of the subject liquid nitriding process is substantially less expensive than conventional liquid nitriding processes. As a result of this greatly improved economy, the subject liquid nitriding process may be used for many applications in which conventional liquid nitriding processes would be impractical due to the high cost Considerable variation may be employed in making up the subject liquid nitriding salt bath compositions of the present invention, although the bath compositions must contain alkali metal and/ or alkaline earth metal cyanide or carbonate salts which will react with the decomposition products of urea to form cyanate compounds in the bath. The cyanate compounds so formed provide the requisite nitriding activity in the bath for case hardening ferrous alloy materials in accordance with the subject liquid nitriding process. Cyanide salts, particularly sodium cyanide and potassium cyanide, and carbonate salts, particularly sodium carbonate, potassium carbonate'and barium carbonate, or various mixtures of these salts with other relatively inexpensive salts, such as alkali metal and/ or alkaline earth metal chlorides, are preferred for use in accordance with the presentinvention in making up the subject liquid nitriding salt bath compositions. However, other cyanide and carbonate salts of alkali metals or alkaline earth metals, such as lithium, calcium or strontium may also be used in accordance with the subject process. The chloride salts, such as potassium chloride, sodium chloride, barium chloride and calcium chloride, may be used in the salt bath compositions of the present invention to decrease the cost and melting point of the bath.

The subject salt bath compositions should contain a sufficient amount of the above-mentioned cyanide and/ or carbonate salts so that upon the addition of urea to the bath the resultant bath composition will have a cyanate salt concentration ranging between about 10% to 60%, by weight, of the bath. In most applications utilizing the subject liquid nitriding process, the resultant bath composition should preferably contain, by weight, between about 25% to 50% cyanate salts. Also, the subject salt bath compositions should fuse at or below the desired nitriding temperature to provide a molten bath in which the ferrous alloy article may be immersed to obtain the desired case hardening effect. A salt bath composition which will fuse in a temperature range between about 850 F. to about 1100 F. is suitable for most applications utilizing the liquid nitriding process of the present invention. Also, a bath operating temperature ranging between about 950 F. to about 1100 F. may be suitably employed for case hardening most ferrous alloy articles, such as an SAE 1020 or SAE 4027 steel article, by the subject process. However, a bath operating temperature ranging between about 1000 -F. to about 1050 F. is generally preferred.

By way of example, a suitable liquid nitriding salt bath may be prepared in accordance with the present invention using a salt mixture which consists, by weight, of about sodium cyanide and about 25% potassium cyanide. Sodium cyanide melts at about 1045" F. and potassium 3 cyanide melts about 1170 F., whereas the 75-25 mixture has a melting point of about 950 P. which is sufficiently low for use in the above-mentioned operating temperature ranges. This cyanide salt mixture will react upon heating with the decomposition products of urea to provide a resultant nitriding bath having a sufliciently low melting point which will minimize distortion of the ferrous alloy articles which are nitrided therein.

Several methods may be employed to introduce the urea into the subject salt bath compositions, such as the aforementioned sodium cyanide-potassium cyanide salt mixture, although the preferred method is to introduce powdered crystalline urea directly into the salt mixture after it has been fused to provide a molten bath. However, since urea rapidly decomposes and generates ammonia at temperatures above its melting point of 270 F., the urea is preferably added slowly to the molten salt mixture to minimize the evolution of ammonia and prevent serious spattering of the molten salt bath mixture. Preferably, the molten bath is agitated when the urea is added to obtain better dispersal of the urea in the bath. This may be conveniently accomplished by bubbling dry air through the bath. 1 have found that the addition of urea directly to the subject molten salt bath compositions of the present invention results in approximately one-fifth recovery of the urea appearing as a weight gain in the resultant salt bath composition.

The second method of introducing urea into the subject salt bath compositions of the present invention, such as the aforementioned sodium cyanide-potassium cyanide mixture, is to simultaneously fuse the'urea with the salt mixture. However, this method generally results in a lower recovery than the direct addition method, since only about one-tenth of the weight of urea added to the bath appears as a weight gain in the resultant salt bath composition. Other methods which may be employed in accordance with the presentinvention to introduce urea into the subject salt'bath compositions are to melt the urea separately and add it to the fused salt mixtures or to use a prepared salt containing urea directly to the fused salt mixtures.

Uponthe addition of urea to a salt bath containing cyanides, such as sodium cyanide and/or potassium cyanide at temperatures above 270 F. ammonia is evolved and cyanic acid is formed. However, the cyanic acid rapidly trimerizes under these conditions to form the more stable cyanuric acid, or it reacts with excess urea to form biuret in accordance with the following reactions:

+ HCNO cyanic acid (NH2)2C O NH urea ammonia 3HCNO (HON O )a cyanic acid cyanuric acid (a trimer of cyanic acid) HCNO (NH2)2CO NH(C ONHz): cyanic acid urea biuret However, when urea is added in small amounts to a relatively large amount of an excess of molten cyanides, it is believed that little or no biuret is formed, since excess urea is not present, and the molten cyanide salts react with cyanic acid or cyanuric acid to generate cyanate salts, such as sodium cyanate and potassium cyanate, in' accordance with the following reactions:

N30 N H N O Sodium cyanide cyanic acid NaC N O H N sodium cyanate hydrogen cyanide K C N H C N 0 potassium cyanide cyanic acid K C N O H C N potassium cyanate hydrogen cyanide At the temperatures generally employed in the subject process, the hydrogen cyanide which is generated in the molten salt bath is rapidly oxidized to form water, nitrogen and carbon dioxide, thereby preventing the evolution of hydrogen cyanide, as such from the bath.

EXAMPLE I About 25 pounds of powdered crystalline urea was directly and slowly added to 95 pounds of a fused cyanide salt mixture consisting, by weight, of about sodium cyanide and about 25% potassium cyanide at a temperature of about 1000 F., and resulted in a 5-pound weight gain in the bath due to the formation of the relatively heavy sodium cyanate and potassium cyanate compounds in the bath. An analysis of the resultant l00-pound salt bath mixture disclosed that the bath contained, by weight, about 69% cyanide salts, about 24% cyanate salts with the balance consisting of sodium and potassium carbonate salts. An SAE 1020 steel sample test bar, hereinafter designated as Sample A, was immersed in this salt bath composition for about two hours at a temperature of about 1050 F. After this treatment, Sample A was removed from the bath and subjected to an immediate water quench. Sample A which was thus treated exhibited a hardened outer case or white layer having a thickness of about 0.0005 inch and a Rockwell C hardness of about 72-46. X-ray diffraction analysis determined that the composition of the case or white layer consisted primarily of epsilon-iron nitride (E-Fe N-Fe N) and iron nitride (Fe N) with a smaller amount of iron carbide (Fe C) also being present. A diffusion zone was also discernible immediately adjacent the outer white layer which showed that the penetration of Fe N needles into the core of Sample A to a depth of about 0.017 inch to about .020 inch.

As previously mentioned, in addition to the use of urea in salt bath compositions containing cyanide salts, as set forth in Example 1 above, the present invention also contemplates the use of urea in salt bath compositions containing alkali metal and/ or alkaline earth metal carbonate salts. These carbonate salts in a fused salt mixture will also react with the decomposition products of urea to form cyanate compounds which provide the desired nitriding activity in the subject liquid nitriding bath compositions of the present invention, Moreover, these carbonate-containing salt bath compositions may be even less expensive than the cyanide-containing baths, since the carbonate salts are generally less expensive than the cyanide salts. The chemical reactions involved when urea is added to a salt mixture containing sodium car bonate are as follows:

(NHmCO HONO NH:

urea cyanic acid ammonia NaaC 0: 2HCNO sodium carbonate cyanic acid 2NaCNO E20 00:

sodium cyanate water carbon dioxide EXAMPLE II fusion temperature of the bath, since sodium carbonate melts at approximately 1560 F. A sufficient amount of urea was added to this fused salt mixture at about 1000 F. to provide a resultant bath composition containing, by weight, about 30% sodium cyanate, about 15% sodium carbonate, about 50% potassium chloride and about 5% sodium cyanide was also formed. An SAE 1020 steel sample test bar, hereinafter designated as Sample B, and

having the same dimensions as Sample A, was immersed in this salt bath composition for about two hours at a temperature of about 1050 F. After this treatment, Sample B was removed from the bath and subjected to an immediate water quench.

Sample B which was thus treated exhibited a hardened outer case or white layer having a thickness of about 0.0005 inch and a Rockwell C hardness of about 24-50. X-ray difiraction analysis determined that the composition of the case or white layer consisted primarily of epsilon-iron nitride (E-Fe N-Fe N) and iron nitride (Fe N) with a smaller amount of iron carbide (Fe C) also being present. A diffusion zone was also discernible immediately adjacent the white layer which showed the penetration of Fe N needles into the core of Sample B to a depth of about 0.017 inch to 0.018 inch Thus, the resultant case hardening elfect was very similar to that imparted to Sample A using the same process conditions of temperature, immersion time and cooling treatment as set forth in Example I.

Of course, it should be appreciated that the resultant case hardening elfect which may be imparted to a ferrous alloy article by the subject process may vary considerably depending on the results desired and the process conditions employed, such as article composition, bath composition, bath operating temperature and exposure time of the article in the bath. Moreover, nitriding bath compositions may be prepared in accordance with the present invention which initially contain both cyanide and carbonate salts prior to the addition of urea. Generally, the thickness and hardness of the case will increase with increased exposure time. However, an exposure time between about one hour to five hours is generally suitable for most applications utilizing the subject liquid nitriding process.

Also, higher process temperatures within the aforementioned operating temperature range of 950 F. to 1100 F. usually increases the hardness of the case. In addition, higher process temperatures generally improve the adherence of the case and the fatigue life of the article due to the increased diffusion rate of nitrogen into the core of the article below the outer case or white layer. Generally, the hardness and thickness of the case is increased with increased concentrations of cyanate salts in the subject liquid nitriding bath compositions, due to the increased diifusion rate of nitrogen into the ferrous alloy article. Also, the formation of iron carbide generally decreases as cyanate salt concentration in the subject bath compositions is increased.

In order to demonstrate the effect of cyanate concentration in the bath composition on case properties imparted to a ferrous alloy article, three difierent SAE 1020 samples were treated in accordance with the subject liquid nitriding process using the 75-25 sodium cyanide-potassium cyanide salt bath mixture as set forth in Example I, each bath having a different concentration of cyanate compounds. The concentration of the sodium cyanate and potassium cyanate compounds formed in the separate baths upon the addition of different amounts of urea was about 35% and 50%, by weight, of the resultant bath compositions and each of the samples was immersed for about two hours in a different bath at a temperature of about 1050" F. and then subjected to a water quench. It was found that a bath containing about 35 by weight, of cyanate salts provided optimum nitrogen penetration into an SAE 1020 steel sample. However, porosity of the white layer generally increased with cyanate concentration although it was not substantial in the 25% to 35% cyanate-containing bath compositions. The effects of cyanate concentration in the above-mentioned baths on the case hardening properties imparted to these three samples are listed in Table I below:

Table I White White Cyanate Concentration Layer Layer Depth of (Percent by Weight) Thickness Hardness 1n Diffusion (In.) Rockwell Zone (In.)

0 Units Table II below shows the composition of the case formed on these three samples in terms of relative amounts of the case constituents as a function of cyanate concentration in the baths:

Table II Cyanate Concentration (Percent, by Weight) In carrying out the subject liquid nitriding process, dry air preferably is bubbled through the subject salt bath compositions to agitate the bath and reduce the process cycle time. Tests have shown that the subject salt bath compositions of the present invention maintain their nitriding ability over relatively long periods of time, although those baths containing primarily cyanides have a longer eifective bath life than baths containing primarily carbonate salts. Of course, the nitriding activity of the subject bath compositions may be rejuvenated at any time during the process by the addition of more urea. Any suitable cooling process may be employed in accordance with the present invention to cool the article after it has been removed from the molten salt bath. For instance, water quenching, oil quenching or air cooling may be employed to cool the case hardened articles, depending on size, shape and compositionof the article.

In order to demonstrate the improvements in wear resistance and fatigue life which may be imparted to a ferrous alloy article by the subject liquid nitriding process, wear and fatigue tests were conducted on the aforementioned test bar Samples A and B, which were case hardened in accordance with the subject process, an untreated SAE 1020 steel test bar, hereinafter designated as Sample C, and an SAE 1020 steel test bar, hereinafter designated as Sample D, which was case hardened in accordance with a conventional liquid nitriding process. All of the samples were subjected to the same test conditions.

However, Sample D was case hardened at a temperature of 1050 F. for about two hours using a conventional liquid nitriding salt bath composition consisting, by weight, of about 45% of a -25 mixture of sodium cyanide and potassium cyanide, about 30% of a 75-25 mixture of sodium cyanate and potassium cyanate, with the balance being about 10% sodium carbonate and 15% potassium chloride. Sample D was then removed from the bath and subjected to an immediate water quench. Hence, the same process conditions of time, temperature and cooling treatment used to case harden Samples A and B in accordance with the subject process were used to case harden Sample D, except, of course, for the liquid nitriding bath composition containing the relatively expensive cyanate salts in the instance of Sample D. The white layer imparted to Sample D by this conventional liquid nitriding process had a thickness of about 0.0004 inch, having a Rockwell C hardness of about 27-45 and a nitrogen diffusion zone depth of about .017 inch to about .018 inch.

All of the samples had the same dimensions and the wear tests were conducted using a constant friction-rotating wheel-test procedure wherein each sample was held stationary against a 7.68 inch diameter cast iron wheel rotating at 145 rpm. for about 18 hours at a mean surface load of about 500 p.s.i. The amount of weight loss by the sample test bars at the end of this period under these test conditions provides an indication of the relative wear resistance of the sample and the results of these wear tests are set forth in Table III below:

Table III Weight loss in milligrams Sample A 6.0 Sample B 5.8 Sample C 179.5 Sample D 6.2

From the foregoing test results it will be noted that Samples A and B, which were case hardened in accordance with the subject process, demonstrated improved Wear resistance over Sample D, which was case hardened using the aforementioned conventional liquid nitriding process.

Fatigue tests were also conducted on Samples A, vB, C and D using the same test conditions in a conventional Budd-Tatn-all-Krouse Rotating Beam Testing Machine. This machine was a 200-inch-pound cantilever loaded type, and all of the samples were flexed at about 6,000 cycles per minute to obtain a maximum stress of 75,000 pounds per square inch at the midpoint of the sample test bars. The samples were subjected to these test conditions until failure occurred and the number of cycles obtained for each sample prior to failure indicates the relative fatigue life of the sample. The test results under the above-mentioned test conditions are listed in Table IV below:

Table IV Number of cycles at which failure occurred Sample A 536,000 Sample B 2,500,100 Sample C 15,700 Sample D 207,100

From the foregoing test results it will be noted that the fatigue life of Samples A and B was substantially improved by the subject liquid nitriding process of the present invention when compared to the results obtained with Samples C and D. Also, in the instance of Sample B, the improvement obtained was substantially greater than that imparted to Sample D which was case hardened by the aforementioned conventional liquid nitriding process.

While I have described my invention in terms of certain preferred embodiments and specific examples thereof, it is not intended to be limited thereby, and it should be understood that other variations may be apparent to those skilled in the art and are within the intended scope of the invention as defined by the following claims.

I claim:

1. A method of liquid nitriding a ferrous alloy article, said method comprising the steps of adding urea to a fused salt mixture including at least one salt selected from the group consisting of alkali metal cyanides, alkali metal carbonates, alkaline earth meta-l cyanides, alkaline earth metal carbonates, said urea decomposing in said mixture and reacting with said salt to form a resultant nitriding salt bath which includes, by weight, between about to about 60% cyanate compounds, immersing said article in said bath for a sufficient time and at a suitable temperature to form a nitride case thereon, removing said article from said bath, and subsequently cooling said article.

2. A method of forming a wear-resistant nitride case between about 850 F. to 1100 F. and including at least one salt selected from the group consisting of alkali metal cyanides, alkali metal carbonates, alkaline earth metal cyanides and alkaline earth metal carbonates, said urea decomposing in said mixture into its decomposition products, said decomposition products reacting with said salt to form a resultant nitriding salt bat-h containing, by weight, between about 20% to about 50% cyanate compounds which serve as the source of nascent nitrogen in said bath, immersing said article in said bath for a sufficient time and at a temperature ranging between about 950 F. to about 1100 F. to form a nitride case on said article, removing said article from said bath, and subsequently cooling said article.

3. A method of liquid nitriding a ferrous alloy article comprising the steps of adding urea toa fused salt mixture having a melting temperature ranging between about 850 F. to about 11 00" F. and including at least one salt selected from the group consisting of lithium cyanide,

sodium cyanide, potassium cyanide, lithium carbonate,

ucts, said decomposition products reacting with said saltto form a resultant nitriding salt bath containing, by weight, between about 20% to about 50% cyanate compounds which provide the bath with its nitriding activity, immersing said article in said bath for about one to five hours at a temperature ranging between about 1000 F. to about 1050 F. to form a nitride case on said article, removing said article from said bath, and subsequently cooling said article.

4. A method of case hardening a ferrous alloy article to improve the fatigue life and wear resistance of said article, said method comprising the steps of adding urea to a fused salt mixture which includes sodium cyanide and potassium cyanide, said urea decomposing in said mixture into its decomposition products which react with said cyanides to provide a resultant salt bath containing, by weight, between about 10% to about 60% of sodium cyanate and potassium cyanate which act as the source of nascent nitrogen in said bath, immersing said article in said 'bath for a sufficient time at a temperature ranging between about 950 F. to about 1100 F. to form a nitride case thereon, removing said article from said path, and subsequently cooling said article to harden and compressively stress said case, thereby improving the fatigue life and wear resistance of said article.

5 A method for liquid nitriding a ferrous alloy article comprising the steps of providing a fused salt mixture consisting, by weight, of about 75% sodium cyanide and about 25% potassium cyanide, adding a sufiicient amount of urea directly to said fused salt mixture to provide a resultant molten salt bath containing, by weight, between about 20% to about 50% of sodium cyanate and potassium cyanate, bubbling air through said bath to agitate it, immersing said article in said bath for about one to five hours at a temperature ranging between about 1000 F. to about 1050 F. to provide a nitride case on said decomposing in said mixture into its decomposition prod ucts which react with said sodium carbonate to provide a resultant salt bath containing, by weight, between about 10% to about 60% of sodium cyanate which acts as the source of nascent nitrogen in said bath, immersing said article in said bath for a sufiicient time at a temperature ranging between about 950 F. to about 1100 F. to form a nitride case thereon, removing said article from said bath, and subsequently cooling said article to harden and compressively stress said case, thereby improving the fatigue life and wear resistance of said article.

7. A method for liquid nitriding a ferrous alloy article comprising the steps of providing a fused salt mixture consisting, by weight, of about 50% sodium carbonate and about 50% potassium chloride, adding a sufiicient amount of urea directly to said fused salt mixture to provide a resultant molten salt bath containing, by weight, about 20% to about 50% of sodium cyanate, bubbling air through said bath to agitate it, immersing said article in said bath for about one hour to about five hours at a temperature ranging between about 1000 F. to about 1050" F. to form a nitride case on said article, removing said article from said bath, and subsequently cooling said article.

1 0 References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 12/1930 Great Britain. 9/ 19-35 Great Britain.

DAVID RECK, Primary Examiner.

15 C. N. LOVELL, Assistant Examiner. 

4. A METHOD OF CASE HARDENING A FERROUS ALLOY ARTICLE TO IMPROVE THE FATIGUE LIFE AND WEAR RESISTANCE OF SAID ARTICLE, SAID METHOD COMPRISING THE STEPS OF ADDING UREA TO A FUSED SALT MIXTURE WHICH INCLUDES SODIUM CYANIDE AND POTASSIUM CYANIDE, SAID UREA DECOMPSOTING IN SAID MIXTURE INTO ITS DECOMPOSITION PRODUCTS WHICH REACT WITH SAID CYANIDES TO PROVIDE A RESULTANT SALT BATH CONTAINING, BY WEIGHT, BETWEEN ABOUT 10% TO ABOUT 60%D OF SODIUM DYANATE AND POTASSUM CYANATE WHICH ACT AS THE SOURCE OF NASCENT NITROGEN IN SAID BATH, IMMERSING SAID ARTICLE IN SAID BATH FOR A SUFFICIENT TIME AT A TEMPERATURE RANGING BETWEEN ABOUT 950*F. TO ABOUT 1100*F. TO FORM A NITRIDE CASE THEREON, REMOVING SAID ARTICLE FROM SAID PATH, AND SUBSEQUENTLY COOLING SAID ARTICLE FROM SAID PATH, AND SUBSEQUENTLY COOLING SAID ARTICLE TO HARDEN AND COMPRESSIVELY STRESS SAID CASE, THEREBY IMPROVING THE FATIGUE LIFE AND WEAR RESISTANCE OF SAID ARTICLE. 